Passengers travelling on commercial air planes in the United States and other countries have to undergo a security screening at the airport. All baggage travelling with the passenger is also checked. Two types of passenger luggage are distinguished: carry-on luggage that stays with the passenger at all times and checked-in luggage that is handled by the airline. For carry-on luggage, security screening is conducted as quickly as possible in an effort to limit the amount of time it takes for passengers to pass into the airport terminal. As a result, the systems used at the checkpoint are fairly limited because they usually generate only a limited number of projection views (less than 10 views) of the object for visual inspection by a trained human operator. The operator is inspecting the two-dimensional (2D) images for contraband such as weapons or explosives. Because of the limited number of views, objects are usually overlapping in the image making the identification of threats a difficult task for operators and also for software algorithms.
In contrast, for checked baggage, advanced explosion detection systems (EDS) are used that produce high resolution three-dimensional (3D) images and have built in threat detection algorithms that search for hidden contraband automatically. Specifically, x-ray computed tomography (CT) scanners have been used in airports for screening checked baggage to detect whether explosives or other contraband are present within the items. Conventional CT baggage scanners rotate a single-beam x-ray tube and a curved detector in a circular gantry rapidly around a center axis to obtain the 700 to 1,000 2D views needed for 3D reconstruction by the filtered back-projection (FBP) method. In such a system, the baggage items are carried on top of a conveyor belt placed near the central axis of rotation (Z axis), along which the conveyor belt moves as the gantry is rotating. The Z travel length of the baggage irradiated by the x-rays during each rotation is proportional to the moving speed of the conveyor belt and the time period of the rotation of the gantry that holds the x-ray source and detector array. For instance, a state-of-the-art rotating gantry CT may complete two to four revolutions per second.
Conventional CT baggage scanners typically utilize a fan beam in the x-y plane and a single row detector, limiting the volume resolution. Another alternative is to utilize multi-row detectors and sophisticated cone-beam image reconstruction algorithms, which can, in principle, offer a finer volume image reconstruction. Nevertheless, a conventional high throughput CT scanner necessitates a fast-rotating gantry to minimize the baggage travel in Z direction during each rotation of the gantry.
This fast rotation creates several reliability and imaging problems, however. For instance, these rotating gantries are characterized by a large, heavy rotating ring that requires significant space and is highly susceptible to breakdowns caused by the high G forces generated by its rotation. The resulting mechanical wear and tear causes high down-time and necessitates expensive maintenance. The G forces also limit scanning speed, thereby reducing throughput capacity. Furthermore, outside the isocenter, the fast rotation of the gantry causes motion-induced blurring, and this blurring increases as the ball bearings supporting the gantry wear down. Such blurring has been recognized as a leading cause of false alarms. The TSA has reported that the cost of the second and third tier inspection procedures to resolve false alarms costs several hundred million dollars annually.
Accordingly, a stationary gantry CT system that can reduce or eliminate the drawbacks associated with a rotating gantry and that can be built and arranged in a custom geometry to fit the optimum arrangement for the objects being inspected would be advantageous.